The paper describes the principal characteristics of SECANT, a system for the Separation and Control of Aircraft using Nonsynchronous Techniques. This cooperative, transponding collision-avoidance system, designed to be compatible within the entire aviation community, is capable of accommodating the dense air traffic anticipated for the 1980s and beyond. It makes available to the pilot evasion or escape maneuvers in any direction-vertical, horizontal, or a combination.SECANT helps the pilot to avoid mid-air collisions by transmitting probes and receiving replies with a 1 μs pulse at 1000 pulses/s on 24 different frequencies. Various discriminants are used to eliminate undesired signals, and the false alarm rate is near zero.The capabilities of each of the following modular equipments are discussed: Remitter, Proximity Warning Indicator, Vicinity Traffic Finder, Collision-Avoidance System, and Traffic-Monitoring System. The correlator, which transmits a randomly selected frequency probe and, when a corresponding frequency probe is received, retransmits an appropriate reply, is described. The theory of operation is presented, and development plans are outlined.